Frequency-domain optical interferometry imaging apparatus and method for astigmatistic bi-focal illumination imaging of an eye

ABSTRACT

Embodiments of the present invention provide a method and apparatus for frequency-domain optical interferometry imaging. Embodiments of the invention include an apparatus comprising a line-shaping optical element for directing optical radiation into a line illumination, an imaging optical element for receiving optical radiation comprising radiation reflected from a target sample and a reference point associated with the target sample, and a detection unit for measuring common path interferences between a plurality of reflections from the target sample and the reference point. Embodiments of the invention include a method comprising directing radiation into a line illumination, directing the line illumination towards a target sample, receiving radiation reflected from the target sample at a detection unit, and measuring common path interferences between a plurality of reflections at the target sample and a reference point.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a U.S. National Stage Application under 35 U.S.C. §371 of International PCT Application No. PCT/GB2017/050934 titled“OPTICAL INTERFEROMETRY APPARATUS AND METHOD,” filed Apr. 3, 2017, whichclaims priority to United Kingdom Application No. 1605616.0, filed Apr.1, 2016, each of which is incorporated by reference herein in itsentirety.

The present invention relates generally to the field of opticalinterferometry. The present invention is applicable, but notexclusively, to the imaging of complex coating structures and biologicaltissues such as that of the eye.

BACKGROUND

Optical interferometry is a technique in which the interferenceproperties of light are used to obtain high resolution images and/ormeasure properties in a diversity of materials.

Examples of techniques which use the interferometry method includeoptical coherence tomography (OCT) and optical coherence reflectometry(OCR). OCT is a non-invasive imaging technique that can uses light toproduce sub-surface, cross-sectional images from within opticalscattering media. OCR is a similar measurement technique that processesthe interference patterns of reflected waves from a sample to determinesample structural characteristics. Due to its ability to achievemicrometre resolution, the clinical adoption of OCT in ophthalmology inparticular is now well established and commercial systems are in routineuse for research and clinical applications.

It is desired to improve upon one or more of scanning speeds, achieveddisplacement or motion sensitivity, and resolution of interferometrysystems.

It is an object of embodiments of the invention to at least mitigate oneor more of the problems of the prior art.

BRIEF DESCRIPTION OF DRAWINGS

One or more embodiments of the invention will now be described, by wayof example only, with reference to the accompanying figures, in which:

FIG. 1a shows an OCT or OCR system in accordance with an embodiment ofthe present invention;

FIG. 1b shows an OCT or OCR system in accordance with an embodiment ofthe present invention;

FIG. 2 shows an OCT or OCR system in accordance with another embodimentof the present invention;

FIG. 3 shows a schematic of an embodiment of the present invention;

FIG. 4 shows a schematic of another embodiment of the present invention;

FIG. 5 shows a schematic illustration according to an embodiment of thepresent invention;

FIG. 6 shows an example image produced according to an embodiment of thepresent invention;

FIG. 7 shows an example image produced according to an embodiment of thepresent invention;

FIG. 8 shows an example image produced according to an embodiment of thepresent invention;

FIG. 9 shows an example image produced according to an embodiment of thepresent invention; and

FIG. 10 shows an example image produced according to an embodiment ofthe present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

FIG. 1a shows a common-path line-field OCT system according to anembodiment of the present invention. The system in FIG. 1a comprises animaging lens or lens system 170, a target sample 180, and a detectionunit 190. The detection unit 190 may be arranged to measure common pathinterferences between a plurality of reflections from the target sample180 and at least one reference point. In some embodiments, the imaginglens 170 may be a spherical or aspherical refractive or reflective lensor lens system i.e. comprising more than one lens. In some embodiments,the detection unit 190 may be a spectrograph. The detection unit 190 maycomprise a line-imaging spectrograph system. The spectrograph 190 may bearranged to produce spectres spatially resolved in one lateraldimension. In some embodiments, the target sample 180 may be illuminatedby an external source of optical radiation. The optical radiation may bea line illumination. The target sample 180 may be arranged relative tothe imaging lens 170 such that optical radiation reflected from thetarget sample 180 is directed by the imaging lens 170 towards thedetection unit 190. Depending on various characteristics of the targetsample 180, after being directed by the at least one imaging lens 170the radiation is reflected or scattered back from at least two points onthe surface of, within, or external to the target sample 180. The pointsof reflection comprise at least one reference point and one samplepoint. As will be explained later, the interference pattern given by thecombination of the reflected wave fronts from at least these two pointsprovides information on the characteristics of the target sample 180,and is thus measured at the detection unit 190.

FIG. 1b illustrates a common-path line-field OCT system 100, inaccordance with another embodiment of the present invention. The system100 comprises a line-shaping optical element for directing light into aline-field focus 150, the imaging lens element 170, and the detectionunit 190 as previously described unless stated otherwise. The system 100also optionally comprises a wave guide 120, a collimator lens 130, andan optical filter 140. In some embodiments, there may exist a furtherimaging element 171. In some embodiments, the line-shaping opticalelement 150 may be a line-field gate element that is insteadincorporated into imaging optical elements. In some embodiments theremay further exist a beam splitting means 160. In some embodiments, thedetection unit 190 may include one or more processing means, such as oneor more processing devices, arranged to perform further processing onthe images received by the detection unit 190. Included in FIG. 1b isalso a source of optical radiation 110 and the target sample 180. Insome embodiments, the system 100 is arranged such that illuminationprovided by the source of optical radiation 110 is first directed viathe wave guide 120 towards the collimator lens 130. However, it shouldbe realised that the wave guide 120 and collimator lens 130 may beabsent in some cases, for example if the source of illumination isprovided by a suitable light source. A suitable light source may be onewhich provides directed or collimated light such as a light emittingdevice which may be, for example, one or more LEDs.

In some embodiments, the optical radiation 110 may compriselow-coherence light, or broadband EM radiation within one or more of avisible, infra-red, and/or ultraviolet spectrums. In some embodiments,the optical radiation 110 may be provided by multiple sources of opticalradiation. In some embodiments, the optical radiation 110 may compriseradiation provided by multiple sources of optical radiation combinedtogether. For example, the optical radiation 110 may comprise combinedradiation beams each having a differing phase or optical path length. Insome embodiments, the optical radiation 110 may comprise multipleradiation beams provided by a single source of optical radiation,wherein each beam has a differing phase or optical path length, and themultiple radiation beams are split from the single source of opticalradiation via a beam splitter or by any other suitable method. In someembodiments, the wave guide 120 may comprise an optical fibre 120.

After passing through the collimator lens 130, collimated radiation isthen passed through the filter 140. The filter 140 is arranged tocontrol spectral intensities of the radiation passing therethrough. Insome embodiments, the filter 140 is bandpass in nature and only permitswavelengths between first and second wavelengths such as 700-1000 nm topass through. In some embodiments, multiple filters may be used incombination.

In some embodiments, radiation having exited the filter 140 is thendirected towards the line-shaping optical element 150. In someembodiments, the line-shaping optical element 150 may be a cylindricalor acylindrical lens. In other embodiments, the line-shaping opticalelement 150 may be a line-field gate. In some embodiments, theline-shaping optical element 150 may be incorporated into the entranceof the detection unit 190. Shaping the radiation into a lineillumination via the line-shaping optical element 150 allows theexposure time to be significantly reduced when imaging, which may resultin one or more of quicker B-scan speeds, reduced distortion, and fewerimage artefacts. Motion artefacts in particular can be reduced using theline-field focus. In addition, the line-field focus gives two spatialdimensions when imaging without the need for moving parts, thussimplifying the overall imaging mechanism. Finally, phase informationmay also be preserved using this method of imaging. Preserving phaseinformation allows for improved sensitivity of measurement, wherein thesensitivity refers to either changes in optical displacement, such asthickness of the target sample with time, the difference in thicknessbetween laterally adjacent points, difference in height in laterallyadjacent points (i.e. profilometry) or change in material refractiveindex spatially or temporally.

In some embodiments, the line illumination is then directed towards thetarget sample 180 via the imaging lens element 170. In some embodiments,the imaging lens 170 may be a spherical or aspherical refractive orreflective lens or lens system. Depending on various characteristics ofthe target sample 180, after being directed by the imaging lens element170 the radiation is reflected or scattered back from at least twopoints on the surface of, within, or external to the target sample 180.The points of reflection comprise at least one reference point and onesample point. The interference pattern given by the combination of thereflected wave fronts from these two points provides information on thecharacteristics of the target sample, and is thus measured at thedetection unit 190.

In other embodiments, the setup may include the use of a beam splittingmeans 160 to allow the same path to be used for illumination andimaging. Use of a beam splitting means 160 also allows the apparatus tobe adapted for conventional interferometry situations. Thebeam-splitting means 160 directs at least a portion of the illuminatingradiation towards the imaging lens element 170 and the target sample180, and at least a portion of the reflected radiation reflected from atleast two points on the surface of, within, or external to the targetsample 180 is returned to the detection unit 190 via the same beamsplitting means 160 along the same path. In some embodiments, theimaging lens element 170 may be a spherical or aspherical refractive orreflective lens or lens system. The imaging lens element 170 is arrangedto produce an image of the target sample 180 at the detection unit 190.

In some embodiments, at least one reference point consists of an areaexternal to the target sample 180, for example a reference mirror or acoverslip adjacent to the target sample. In other embodiments, at leastone reference point comprises an area positioned within the targetsample 180. In still other embodiments, at least one reference point isan area on the surface of the target sample 180.

In some examples, the at least one reference point is an area on theimaging lens element 170 itself, i.e. via a Mirau setup.

In still further embodiments, the interference image is given at leastpartially by the superposition of multiple effective reflected referencesignals, taken from multiple effective reference points. For example, inapplications where the target comprises a human eye the reference pointsmay, for example, correspond to the surface of the tear film, interface,or other reflecting or scattering component of an applied contact lensor other artificial body, or any other appropriate or takenreflection/scatterer within or associated with the eye. In embodimentswhere the source of optical radiation 110 comprises multiple sources,the multiple reference points may correspond to multiple sources ofoptical radiation. The multiple reference points may be taken from atleast one of on the surface, within, or external to the target sample.

Arranging at least some of the reference points to be adjacent or partof the sample allows for a much greater tolerance on the axialpositioning of the target sample when compared with prior art OCTimplementations, and also gives increased stability of interference asmovements of the target sample 180 during the imaging process arecompensated by the equivalent positional shift of the reference point.This reduces the likelihood of motion artefacts in the final image.

In some embodiments, the further imaging element 171 may be a furtherimaging lens element 171 which may be arranged in relation to the firstimaging lens element 170 to receive the reflected beams and to producean image of the target sample 180 at the detection unit 190. In someembodiments, the further imaging lens element 171 may be a spherical oraspherical refractive or reflective lens or lens system.

In some embodiments, the positions of the optical elements 150 and 170are moveably mounted such that the arrangement of focal distances of theoptical elements 150 and 170 can be varied. This allows for a method ofastigmatistic bi-focal illumination of the target sample 180, which willbe explained later.

In some embodiments, the detection unit 190 receives a combination ofthe image and the reference beams, and measures the resultingcommon-path interference. These interferences are further measuredbetween a plurality of reflections at the target sample 180 and thereference point. These measurements are used to produce tomographicimages.

In some embodiments, the detection unit 190 may comprise a spectrograph.In some embodiments, the detection unit 190 may be equipped with a 2Ddetection unit array, where each array element outputs a respectivevalue or signal, such as a CCD or CMOS/sCMOS array. In some embodiments,the detection unit 190 may comprise processing means for processing thereceived images. At the detection unit 190, an array of spectra isobtained from the raw image data received from the reflected andback-scattered radiation from the target sample 180. In someembodiments, the array of spectra is obtained with a uniform wavenumber(frequency) spacing.

In other embodiments, if the spectra are not already in uniformwavenumber spacing, they may be mathematically interpolated via theprocessing means into equal wavenumber spacing. An apodisation ordigital spectrum shaping method may be applied to reduce one or more ofany axial point spread function artefacts/side lobes and/or manipulatethe axial point spread function shape.

A generic Discrete Fourier Transform (DFT) algorithm can then be appliedto each spectra via processing means, thus retrieving the complexA-scan. Multiple A-scans can thus be combined to give a B-scan,comprising a two-dimensional, cross-sectional view of areas within thetarget sample.

In some embodiments, a normalised reference signal taken from a singleinterface measurement is subtracted from the measurements in order toreduce any DC components or static artefacts in the images.

In some embodiments, the thickness of a layer can be found by taking thepeak magnitude points within the A-scan and finding the (approximate)centre of the axial point spread function via the processing means, thusallowing recovery of the position of the interface. In otherembodiments, layer thicknesses can also be extracted using the B-scanmap via image segmentation.

In an embodiment of the present invention, the thickness of the layercan also be found via the processing means using phase information thatis preserved as a result of using a line illumination. In this method,the relative differences in the thicknesses of a layer temporally and/orspatially can be obtained from the complex phase of the interferencedata. The highest value magnitude pixels are first selected from thereceived images at the detection unit 190, and their phases areextracted for use. In some embodiments, the selected pixels may simplybe pixels with a signal-to-noise-ratio (SNR) value above a predeterminedthreshold. Phase difference constants between pixels of different depthsmay be corrected by adding or subtracting an appropriate value and phaseun-wrapping techniques may further be applied to obtain a continuousphase map.

This multi-dimensional data may then be multiplied by λ/4π, where λ isthe appropriate effective central wavelength of light. The result is therelative thickness differences of each layer in the target sample.

In some embodiments, conversion to absolute thickness can be achieved bysubtracting the average value and adding an average thickness calculatedby any methods that return the (approximate) centre of the axial pointspread function.

Using phase information to calculate layer thicknesses allows for higherprecision than obtaining it from axial point spread functions.

FIG. 2 shows a common-path line-field OCT system 200, with theastigmatistic bi-focal illumination setup shown in more detail, inaccordance with an embodiment of the present invention.

In one embodiment of the present invention, flat, collimated wave frontsof illuminating radiation are used for imaging the target sample 180.However this method can present certain issues depending on the natureof the target sample 180. Taking imaging in the cornea as an example,using flat, collimated wave fronts may cause the light to be focused bythe eye onto the retina, increasing incident light intensity on theretina and posing potential health hazards to the patient.

Astigmatistic bi-focal illumination mitigates this problem by bringingthe radiation to a focus at a different point than the retina (such asthat corresponding with the centre of the curvature of the cornea). Thisdecreases the incident light intensity at the retina and any health andsafety limitations become near identical to that of single pointscanning OCT systems. As shown in 200, this can be achieved by varyingthe positions of the line-shaping optical element 150 and/or the imaginglens element 170.

Positioning the line-shaping optical element 150 at position 151 gives aline-shaping optical element focal distance of F_(cy1). Similarly,positioning the imaging lens element 170 at position 172 gives animaging lens element focal distance of F_(obj). In one embodiment,astigmatistic bi-focal illumination is achieved by arranging theposition of optical elements 150 and 170 such that:

$F_{x} = \frac{1}{\frac{1}{F_{obj}} - \frac{1}{F_{obj} + {\Delta\; F}}}$where ΔF is the difference in depth between the two illumination focalpoints in the two lateral dimensions perpendicular to the direction oftravel of light, caused by the interaction of the line-shaping opticalelement 150 and other illuminating lenses e.g. the imaging lens element170. The astigmatistic nature of the line-shaping optical element 150causes light propagating in the two perpendicular planes to havedifferent focal points. When optimised in conjunction with the imaginglens 170, these two focal points form F_(Obj) and F_(ObJ)+ΔFrespectively. F_(x) is the difference in distance between the focalpoint of the line-shaping optical element 150 and the imaging lens 170itself.

FIG. 3 shows an arrangement of the line-shaping optical element 150 andthe imaging lens 170 without a bi-focal astigmatistic illuminationset-up. It can be see that F_(cy1) is the focal distance of theline-shaping optical element 150 and F_(obj) is the focal distance ofthe imaging lens 170. In this embodiment, F_(x) is the same as F_(obj).

FIG. 4 shows an arrangement of the line-shaping optical element 150 andthe imaging lens 170 with a bi-focal astigmatistic illumination set-up.In this arrangement, the line-shaping optical element 150 is positionedsuch that the distance between the line-shaping optical element focalpoint 150 and the imaging lens 170 is given by F_(x). Furthermore, theastigmatistic nature of the line-shaping optical element 150 brings riseto ΔF, as explained above.

FIG. 5 shows the wave fronts of illuminating light at the focus of acommon-path line-field OCT system for conventional and astigmatisticbi-focal line-field OCT, in accordance with one embodiment of thepresent invention. 510 shows collimated light wave fronts as inconventional systems, and 520 shows the equivalent ray tracingrepresentation. 530 shows the light wave fronts when astigmatisticbi-focal illumination is implemented, and similarly 540 shows theequivalent ray tracing representation.

It will be appreciated by the skilled person that applications of theinvention are not limited to biological imaging. As well as biologicaluses, applications of embodiments of the invention include themeasurement of coating thicknesses, such as in industrial applicationsi.e. manufacturing, quality control etc. For example, non-opaquecoatings are used to provide protection and aesthetic enhancement to awide variety of objects—examples include heavy industry clear top coatsin modern car paint systems, varnishing on outdoor wooden objects, andthe highly specialised varnish coatings for valuable works of art.Precise and accurate measurement of the thickness of these coatings arethen useful for quality assurance purposes in particular. It will beappreciated that the usefulness of embodiments of the invention are notlimited to these coatings.

FIG. 6 shows an example OCT image produced according to an embodiment ofthe invention. The target sample imaged in this instance is a multilayerautomotive paint panel. The reference points associated with the targetsample comprised of the surface of a top (uppermost) paint layer only,giving a clear OCT image of the layered structure underneath the toppaint layer. 610 shows an image indicative of the spectral data measuredaccording to an embodiment of the invention, with post-processingapplied. The post-processing applied in this example comprisedinterpolation, Fourier Transform, and logging of the measured spectraldata, however it will be appreciated that other post-processingtechniques may be applied. 620 shows a reference of a single reflectingsurface. 630 shows an enhanced version of 610, created by subtracting620 from 610. With such images, it will be realised that processing oranalytical methods applicable to conventional OCT systems may be appliedto the invention.

FIG. 7 shows an example OCT image produced according to an embodiment ofthe invention. The target sample in this instance comprised aglass-varnish interface. The reference point associated with the targetsample comprised a surface of the varnish. The images shown in FIG. 7were taken at different times as the solvent varnish dried and thinned.

FIG. 8 shows a process of utilising the Fourier phase informationproduced according to an embodiment of the invention to measure changesin varnish thickness (glass-varnish interface position), with spatial(i.e. between different lateral points) and temporal (i.e. over time),and drying rate. 810 shows the pixel chosen vs. space and time. 820shows the Fourier phase of those pixels. 830 is the Fourier phase aftercorrection for the phase differences between pixels. 840 shows the phaseafter unwrapping in both (spatial and temporal) dimensions.

FIG. 9 shows a representation of 840 after converting from phase tothickness. Utilising Fourier phase may provide higher displacementsensitivity than using amplitude information alone.

FIG. 10 shows (displaced for comparison purposes) example thicknessmeasurements, at one position, obtained by finding the centre of thepoint spread function (amplitude information only) and the describedFourier phase method. FIG. 10 shows that the Fourier phase method tohave lower noise.

It will be appreciated that embodiments of the present invention can berealised in the form of hardware, software or a combination of hardwareand software. Any such software may be stored in the form of volatile ornon-volatile storage such as, for example, a storage device like a ROM,whether erasable or rewritable or not, or in the form of memory such as,for example, RAM, memory chips, device or integrated circuits or on anoptically or magnetically readable medium such as, for example, a CD,DVD, magnetic disk or magnetic tape. Such software may be tangiblystored on a computer-readable medium. The computer-readable medium maybe non-stransitory. It will be appreciated that the storage devices andstorage media are embodiments of machine-readable storage that aresuitable for storing a program or programs that, when executed,implement embodiments of the present invention. Accordingly, embodimentsprovide a program comprising code for implementing a system or method asclaimed in any preceding claim and a machine readable storage storingsuch a program. Still further, embodiments of the present invention maybe conveyed electronically via any medium such as a communication signalcarried over a wired or wireless connection and embodiments suitablyencompass the same.

All of the features disclosed in this specification (including anyaccompanying claims, abstract and drawings), and/or all of the steps ofany method or process so disclosed, may be combined in any combination,except combinations where at least some of such features and/or stepsare mutually exclusive.

Each feature disclosed in this specification (including any accompanyingclaims, abstract and drawings), may be replaced by alternative featuresserving the same, equivalent or similar purpose, unless expressly statedotherwise. Thus, unless expressly stated otherwise, each featuredisclosed is one example only of a generic series of equivalent orsimilar features.

The invention is not restricted to the details of any foregoingembodiments. The invention extends to any novel one, or any novelcombination, of the features disclosed in this specification (includingany accompanying claims, abstract and drawings), or to any novel one, orany novel combination, of the steps of any method or process sodisclosed. The claims should not be construed to cover merely theforegoing embodiments, but also any embodiments which fall within thescope of the claims.

The invention claimed is:
 1. An apparatus for frequency-domain opticalinterferometry imaging of a human eye, comprising: a line-shapingoptical element for directing optical radiation into a lineillumination; an imaging lens for receiving optical radiation comprisingradiation reflected from a target sample and a reference point; and adetector for measuring common path interferences between a plurality ofreflections from the target sample and the reference point, wherein thetarget sample is a human eye, and wherein the line-shaping opticalelement and the imaging lens are arranged such that their focaldistances provide for astigmatistic bi-focal illumination of the targetsample.
 2. The apparatus of claim 1, wherein the line-shaping opticalelement is a cylindrical or acylindrical lens.
 3. The apparatus of claim1, further comprising a beam splitting means to direct at least aportion of the radiation.
 4. The apparatus of claim 3, wherein theimaging lens is positioned between the detector and the beam splittingmeans.
 5. The apparatus of claim 1, wherein the apparatus comprises oneor more of a wave guide, a collimator lens, a beam splitting means,and/or an optical filter.
 6. The apparatus of claim 1, wherein theimaging lens comprises a spherical or aspherical lens.
 7. The apparatusof claim 1, wherein the imaging lens comprises a reflective orrefractive lens.
 8. The apparatus of claim 1, wherein the imaging lensis one of a plurality of imaging lenses.
 9. The apparatus of claim 1,wherein the detector comprises a spectrograph.
 10. The apparatus ofclaim 1, wherein the detector comprises one of a CCD, CMOS or sCMOSarray.
 11. The apparatus of claim 1, further comprising multiple sourcesof optical radiation for providing the optical radiation.
 12. Anapparatus as claimed in claim 1 wherein the line-shaping optical elementand the imaging lens system are moveably mounted.
 13. An apparatus asclaimed in claim 12 wherein the line-shaping optical element and theimaging lens system are arranged such that F_(x), the distance betweenthe focal point of the line-shaping optical element and the imaging lenssystem is$F_{x} = \frac{1}{\frac{1}{F_{obj}} - \frac{1}{F_{obj} + {\Delta\; F}}}$where F_(obj) is the focal length of the imaging lens system, and ΔF isthe difference in depth between two illumination focal points in twolateral dimensions perpendicular to a direction of travel of light,caused by the interaction of the line-shaping optical element and theimaging lens system.
 14. A method for frequency-domain opticalinterferometry imaging of a human eye, comprising: directing, using aline-shaping optical element, radiation into a line illumination;directing the line illumination towards a target sample; receiving, viaan imaging lens, radiation reflected from the target sample at adetector; and measuring common path interferences between a plurality ofreflections at the target sample and a reference point, wherein thetarget sample is a human eye, and wherein the line-shaping opticalelement and the imaging lens are arranged such that their focaldistances provide for astigmatistic bi-focal illumination of the targetsample.
 15. The method of claim 14, further comprising formingtomographic images from the measured common path interference readings.16. The method of claim 14, comprising directing the radiation into aline illumination via a cylindrical or acylindrical lens.
 17. The methodof claim 14, comprising directing at least a portion of the radiationvia a beam splitting means.
 18. The method of claim 14, comprisingfurther directing the radiation via a wave guide, a collimator lens,and/or an optical filter.
 19. The method of claim 17, wherein theradiation is directed towards the target sample and the detector via thebeam splitting means.
 20. The method of claim 14, wherein the radiationis directed via a plurality of imaging lenses.
 21. The method of claim14, wherein the radiation is provided by multiple sources.
 22. Themethod of claim 14, comprising moving a position of one or more of theline-shaping optical element and the imaging lens to change a focalpoint of the associated optical element.